If you have a Mac the process is simple. Download your vehicle/s of choice and drag it/them to the Vehicle folder in your Nuvi. My personal favourites are the Helicopter, the American Cop Car and the Model T Ford.

I think there is if you open the vehicle in an image editor like paint, Great your own that way. I do remember trying something like that ages ago. Ill have a look around and see if i can find out what exactly to do